The invention is directed to the delignification of cellulose with oxygen and hydrogen peroxide in acid medium.
Cellulose obtained in a chemical way, such as, e.g., from the sulfite process or which is obtained by the alkaline soda or sulfate process in addition to the main constituent cellulose also contains small amounts of lignin, hemicellulose, and several other constituents. The materials mentioned which accompany cellulose, above all lignin, cause the coloration of the cellulose or products produced therefrom.
In order to produce paper or other products of a high degree of whiteness which are not inclined to yellow from the cellulose, it is necessary to remove by bleaching the accompanying materials remaining after the chemical treatment.
According to the state of the art, the bleaching is carried out in multistep systems with, e.g., chlorine, hypochlorite, oxygen, and hydrogen peroxide. In using chlorine as well as hypochlorite, it is disadvantageous that chlorinated materials are formed which are difficult to break down biologically and in part exhibit mutagenic properties. Vaporization and burning of this waste water is the best method for reducing the waste water problem. The return of the waste water from the bleaching in the chemical purification process of the bleaching of the treatment, however, creates problems because of the corrosion caused by the chloride ions.
Oxygen and hydrogen peroxide today are used exclusively under alkaline conditions as bleaching agent. Therefore, the waste liquor of the bleaching process contains sodium hydroxide liquor and is only returned problem free in a treatment process having sodium as the base. In the production of sulfate cellulose (kraft pulp) in which the processing is with alkaline sulfide solution, this is readily possible. In contrast, the production of sulfite cellulose today is carried out almost exclusively with magnesium or calcium sulfite. In the chemical recovery, a mixture with sodium ions leads to a reduction in melting point in the combustion furnace so that the chemical recovery from the fly ash and/or the degree of efficiency of the plant is disturbed. Therefore, for sulfite cellulose factories based on magnesium, there were developed processes which operate with magnesium oxide as the base. The apparatuses necessary for this purpose, however, are clearly more expensive in comparison to the use of sodium hydroxide liquor, since higher pressures and higher temperatures are necessary.
Therefore, bleaching agents which neither contain or need chloride ions or cations such as Na.sup.+ are desirable for universal use.
This is valid for compounds such as ozone, nitrogen dioxide, and peracetic acid. The bleaching and delignification are carried out with these oxidizing agents under acid conditions so that no cations must be added. In spite of this, previously for economical reasons, none of these bleaching agents has penetrated the industry.
Fossum et al U.S. Pat. No. 4,222,819 is directed to the acid delignification with peroxides, preferably hydrogen peroxide. According to this process, there is connected to the acid treatment an alkaline extraction step in order to remove the lignin that has become soluble.
Since immediately after to the acid treatment with hydrogen peroxide there must follow an alkaline treatment with sodium hydroxide liquor, there is eliminated the possibility of a common vaporization of the waste water of the bleaching step with the waste liquor, e.g., of a magnesium sulfite treatment.
There is described in U.S. Pat. No. 4,410,397 and U.S. Pat. No. 4,427,490 the delignification and bleaching of cellulose in acid medium, and there is proposed the addition of metal additives to improve the effect.
However, it does not appear practical to incorporate metal ions into the bleaching process which subsequently will load the waste water.
The task of the invention was to develop a process for the delignification and bleaching of cellulose in which a return of the waste water from the bleaching into the chemical upgrading process is accomplished as free from problems as is possible.